1. Field of the Invention
The invention relates generally to processing substrates with electron beams. More particularly, the present invention relates to applying electron beams to surfaces of semiconductor substrates to write patterns thereon.
2. Description of the Related Art
In electron beam substrate processing systems, a narrowly converged electron beam is applied to target positions on the surface of substrates to form patterns thereon. Electron beam substrate processing systems generally include an electron gun, electromagnetic lens, and a vacuum chamber for holding the specimen substrate. Electron beam substrate processing systems write a desired pattern on a substrate surface usually coated with electron beam resist by focusing, directing, and blanking the electron beam such that only specified surface areas of the substrate are processed.
Processing substrates generally involves raster-scan and vector-scan processes. According to the raster-scan process, a region of the substrate, also referred to as a field, is sequentially and fully scanned at the same speed, one end to the other, before moving to the next field. The electron beam is applied to the region of the field where a pattern is to be written and blocked, from other regions by a blanking mechanism. According to the vector-scanning process, an electron beam skips areas where no graphic pattern is to be written, and is deflected only to a substrate surface area where a pattern is to be written.
Substrates are often processed with electron beam processing systems where the substrate is placed on a chuck within a vacuum-processing chamber. Such electron beam assemblies usually include a load lock chamber and transfer robot to hold and transfer the substrates onto the chuck for processing. To position the substrate within a preferred radius of the electron beam the chuck and substrate thereon are moved in a horizontal X and Y direction. Once in a desired position the electron beam may be deflected to more accurately position the electron beam onto the substrate.
Substrates may be processed using rotating electron beam substrate processing systems. Rotating electron beam substrate processing systems rotate substrates underneath a narrowly focused electron beam that may also use deflection to accurately position the electron beam on the substrate surface. Similar to the non-rotating electron beam systems, blanking control is generally used to selectively apply the electron beam to a surface region of the rotating substrate surface.
In rotating electron beam substrate processing systems, the substrate is mounted on a rotating spindle assembly. The rotating spindle assembly is coupled to a spindle motor that provides rotational speed to the spindle and therefore the specimen substrate. The rotational velocity and acceleration of the spindle is generally controlled by a controller in communication with the spindle motor. The spindle assembly is generally configured to move in a radial direction relative an electron beam discharge assembly, i.e., electron gun, to allow the electron beam to be precisely positioned on desired surfaces of the rotating substrate. As the substrate is rotated and moved, the electron beam is deflected as needed and applied to the desired regions of the substrate using a blanking control. To correctly apply the proper pattern to the substrate being processed, a pattern clock may be used. Conventionally, the pattern clock is associated with the spindle rotational speed and position of the spindle relative the electron beam such that at calculated times based on the angular rotation and movement of the substrate, a pattern region will be positioned within a radius of the electron beam for processing.
Generally, an optical encoder is used in the control the rotation of the spindle. The optical encoder generally includes an optical reader positioned to read an optical encoder disk. Conventionally, to mechanically couple the optical encoder disk to the substrate, the optical encoder disk is mounted on an end of the spindle shaft distal the substrate. As the spindle motor rotates, the optical reader detects timing marks on the optical disk to determine the speed of rotation. To maximize resolution, the timing marks are usually placed on the outer edge of the encoder disk. The detected timing marks generally provide a rotational velocity signal used as a rotation control signal and as a pattern clock signal for a pattern generation circuit. The rotation of the spindle motor is phase locked to the master clock such that the rotation of the spindle motor is adjusted until the master clock and rotation control signal are in phase. The pattern clock signal is used by the pattern generation circuit to control a blanking mechanism of the rotating electron beam substrate processing system to control the on and off time of the electron beam.
Generally, a greater number of timing marks and a closer spacing therebetween, results in a greater frequency of the rotation control signal, i.e., the pattern clock signal, which is preferable to achieve more accurate circumferential positioning of electron beam recording locations and thereby better pattern resolution. However, many encoders have frequency limitations that limit the upper frequency that can be produced. Generally, a compromise is made on the number or timing marks and spindle rotational velocity to accommodate the limitations of the encoder and achieve acceptable and electron beam circumferential position accuracy. Unfortunately, improved pattern resolution control may be sacrificed for rotational speed control at higher spindle speeds.
Due to the positional distance between the optical encoder disk and the substrate, and torsion on the spindle shaft under rapid acceleration changes, the instantaneous rotational velocity of the optical encoder and substrate may differ. Further, the spindle shaft is subject to structural resonances, bending modes, torsional modes, friction, and the like that contribute to pattern placement errors, e.g., track-to-track phase error of written patterns. Thus, due to such torsional stresses, friction, and structural deflections the pattern generation circuit may be given erroneous timing data causing the pattern position of the substrate surface under the electron beam to be misaligned.
Therefore, a need exists for a method and apparatus to minimize misalignment errors between an electron beam and the substrate pattern target while improving substrate processing pattern resolution.